[llvm-exegesis] Change how the native architecture is determined

Currently the llvm-exegesis native architecture is determined by comparing the
llvm native architecture with X86, so to add a new target would mean adding a
new check. Change this to building up a list of the targets llvm-exegesis
supports then using that, as this means that when adding a new target you just
add the target to the list of supported targets.

Differential Revision: https://reviews.llvm.org/D48778

llvm-svn: 336105
diff --git a/llvm/tools/llvm-exegesis/lib/CMakeLists.txt b/llvm/tools/llvm-exegesis/lib/CMakeLists.txt
index 02eeb75..3c69dea7 100644
--- a/llvm/tools/llvm-exegesis/lib/CMakeLists.txt
+++ b/llvm/tools/llvm-exegesis/lib/CMakeLists.txt
@@ -1,5 +1,6 @@
 if (LLVM_TARGETS_TO_BUILD MATCHES "X86")
   add_subdirectory(X86)
+  set(LLVM_EXEGESIS_TARGETS "${LLVM_EXEGESIS_TARGETS} X86" PARENT_SCOPE)
 endif()
 
 add_library(LLVMExegesis